For several reason, most of which are simply physics. If you throw the ball harder then when I hit it well I hit the ball harder. Balls that are hit harder go further. Last year it's pretty obvious that MLB screwed with the ball, which made it harder for pitchers to spin, which meant that pitches generally went straighter. Balls that are straight are easier to hit than balls that have movement to them, because there is less guesswork in where the ball is going. Balls that are easier to hit can be hit harder, because you aren't fooled as often and you are making solid contact more often. And bats are significantly better now than they were in 1960. They are lighter, which would actually hurt distance in a vacuum, but the upshot of lighter bats with thinner handles is that guy's bat speed on average is higher than it used to be, and faster bats make balls go further. And of course players are bigger and stronger on average than they were back then, and when a bigger, stronger guy makes good contact the ball goes further than when a smaller, not as strong guy does.
There are also things like the average ball park is smaller now than it was back then (just compare the dimensions of Forbes Field to PNC Park). And most players back then would do things like choke up on the bat with two strikes to not strike out whereas players now understand that if you are a power hitter in today's game that's a fool's errand. And so on.